1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an electrophotographic image forming apparatus that uses an image transfer belt to convey an image along an endless loop path for transfer to a recording medium at a transfer gap defined between a pair of pressure members.
2. Discussion of the Background
in electrophotographic image forming apparatuses, such as printers, photocopiers, facsimiles, and multifunctional machines incorporating several of these functions, a color image is produced by combining layers of sub-images formed with toner of different primary colors, cyan, magenta, yellow, and black. Currently, most color electrophotographic systems employ a tandem architecture in which multiple drum-shaped photoconductors are arranged in series along an intermediate transfer belt trained around multiple support rollers.
In tandem color printing, each of the photoconductor drums rotates to pass its outer photoconductive surface through a series of various imaging processes to form a sub-image with toner of a particular primary color, while the intermediate transfer belt travels along an endless loop path upon actuation by a drive roller rotating with the belt support rollers.
During rotation along the belt travel path, the intermediate transfer belt passes an imaging portion or area thereof through a series of primary transfer nips defined between the photoconductor drums and corresponding primary transfer rollers. At each primary transfer nip, a sub-image is transferred from the photoconductive surface to the imaging area with a bias voltage applied to the primary transfer roller pressed against the intermediate transfer belt. As the imaging area proceeds from one nip to another to repeat the primary transfer process, sub-images of different colors are superimposed one atop another to form a composite color toner image on the intermediate transfer belt.
After primary transfer, the intermediate transfer belt advances the imaging area to a secondary transfer nip defined between a pair of secondary transfer members, e.g., a pair of pressure rollers opposed to each other. Simultaneously with the imaging area entering the secondary transfer nip, a recording medium, such as a sheet of paper, also enters the secondary transfer nip to meet the toner image on the belt surface. At the secondary transfer nip, the toner image is transferred from the belt surface to the incoming recording sheet with a bias voltage applied across the pair of transfer rollers pressed against each other, one on the belt side and the other on the sheet side. The recording sheet thus bearing the toner image thereon is then forwarded to fixing and/or other finishing processes to complete one cycle of color image formation.
In such a configuration, it is important for good imaging quality of the tandem color printer to maintain a constant process velocity at which the photoconductor drum passes the photoconductive surface through the series of imaging processes. This is particularly true for the primary transfer process, where major variations in the process velocity create irregularly expanded and compressed areas in resulting images, and even minor ones can result in noticeable variations of toner density in solid prints which should be of a single uniform tone or color.
Theoretically, the process velocity during primary transfer is defined as a velocity of the photoconductive surface relative to a velocity of the intermediate transfer belt passing through the primary transfer nip. Provided that the photoconductor drum rotates at a constant speed, maintaining a constant traveling speed of the intermediate transfer belt is required to maintain a constant process velocity during primary transfer.
There are several factors that contribute to causing variations in the traveling speed of an intermediate transfer belt, one of which arises when the tandem printer processes recording sheets thicker than those used for ordinary printing.
That is, when a thick recording sheet enters the secondary transfer nip, the torque or force required to rotate the belt drive roller abruptly increases corresponding to an increase in the load on the pair of secondary transfer rollers drawing the leading edge of the incoming recording sheet therebetween, in turn causing a temporary decrease in the speed of the intermediate transfer belt. Conversely, when a thick recording sheet leaves the secondary transfer nip, the force required to rotate the belt drive roller abruptly decreases corresponding to a decrease in the load on the pair of secondary transfer rollers expelling the trailing edge of the outgoing recording sheet from therebetween, causing a temporary increase in the speed of the intermediate transfer belt.
Such temporary deceleration and acceleration of the intermediate transfer belt occurring at the secondary transfer nip propagates to the primary transfer nips along the looped belt travel path. When the primary transfer process takes place for a subsequent operational cycle simultaneously with the secondary transfer process, this results in imaging failures due to variations in the velocity of the photoconductive surface relative to that of the intermediate transfer belt.
To address this problem, one conventional image forming apparatus uses a gap adjuster to adjust the distance or transfer gap between a transfer belt and a transfer roller according to a thickness of recording sheet in use. Upon detecting that a recording sheet used is thicker than usual, the gap adjuster widens the transfer gap before the sheet is forwarded to the transfer process. This enables the thick recording sheet to enter and exit the transfer gap without unduly interfering with the transfer roller and the transfer belt, thereby preventing abrupt changes in the load on the transfer roller, as well as concomitant variations in the belt speed and resulting image failures during primary transfer.
A drawback of this method is that the effect of gap adjustment is limited by the extent to which the transfer gap may be widened without affecting the proper functioning of the transfer roller. That is, although widening the transfer gap effectively reduces the load on the transfer roller, it simultaneously means a reduction in pressure applied to the recording sheet entering the transfer gap, and too wide a transfer gap can negate the primary function of the transfer roller and the transfer belt. At the same time, as long as the gap adjuster is required to maintain the width of the transfer gap smaller than the thickness of a recording sheet, the conventional method fails to completely eliminate variations in the belt traveling speed due to the use of thick recording sheets.